For various pants of automobiles and airplanes, various parts structuring gas facilities or water facilities and packages for foods, medical products, etc., it is necessary to inspect leakage caused in the structures, per se, of these items and defects of joint portions and seal portions formed by welding, etc., thereof. Leakage inspection methods have been used, which are illustrated in Table 1, that is, “pressurization foaming method”, “in-water foaming method”, “color check method”, “ultrasonic method”, “differential pressure method”, “gas leak test method”, and the like.
TABLE 1Detection Principle and Quantitativityof Leakage Inspection MethodQuanti-NameDetection principletativityPressurizationSoap water is applied on the outside ofxfoaming methoda specimen and the inside is pressurizedwith gas (air). Bubbles generated froma leak portion are detected.In-waterA specimen is soaked in water and thexfoaming methodinside of the specimen is pressurizedwith gas (air). Bubbles generated froma leak portion are detected.Color checkA color solvent is applied on a specimen.xmethodThe solvent is absorbed at a leak portionand appeared as a maculation and themaculation is detected.UltrasonicWhen there is leakage continuously,xmethodultrasonic waves are generated. Theultrasonic waves are detected with anultrasonic sensor.DifferentialA specimen and a standard body (master)∘pressureare pressurized with air in order tomethoddetect a differential pressure betweenthe specimen exhibiting leakage and themaster exhibiting no leakage.Gas leak testA specimen is entered into a vacuum∘methodchamber and the inside of the specimenis pressurized with a probe gas. Probegas having leaked is detected. Heliumgas is often used as the probe gas.
Oil production sites for various parts and packages, a leakage inspection by taking products that exhibit a flow rate of leakage below a permitted rate as acceptable products has been a mainstream method. On this background, methods as represented by “differential pressure method” and “gas leak test method”, which enable determination of a flow rate of leakage, have been employed widely.
For example, a leakage inspection apparatus of a differential pressure method is disclosed in Japanese Patent Application Laid-open No. H5-296871 (Patent Document 1) and a helium gas leak detection apparatus of a gas leak test method is disclosed in Japanese Patent Application Laid-open No. 2005-140504 (Patent Document 2).
FIG. 3 illustrates a diagrammatic view of a leakage inspection according to a differential pressure method, which is a conventional technique. In the differential pressure method, a tested body and a master from which no leakage occurs are pressurized with a gas, a valve of a pressurization line is closed and thereafter the pressure difference ΔP (Pa) between the tested body and the master is measured after a constant time Δt (s). Supposing the volume inside the tested body is V (m3), the flow rate Q (Pam3s−1) of leakage can be estimated, by Q=V(ΔP/Δt). Since the detection resolution (detection lower limit) of the leakage inspection apparatus using the differential pressure method is about 10−4 Pam3s−1, the method has been mainly used for inspection for parts whose allowable flow rate of leakage is 10−3 Pam3s−1 or more.
FIG. 4 is a diagrammatic view illustrating a gas leak test method, which is another conventional technique. A high pressure probe gas of about 0.5 MPa is filled in a tested body. The tested body is disposed in a vacuum chamber and, when there is a leakage, the probe gas leaking into the vacuum chamber is detected with a gas detector. Herein, a calibration leak providing a known flow rate of leakage is equipped in the gas detector, allowing the flow rate of leakage (Pam3s−1) to be measured.
Helium gas is mainly used for the probe gas and, since the detection resolution of the helium detector (helium leak detector) is about 10−10 Pam3s−1, the gas leak test method has been mainly used for inspecting parts whose minute allowable flow rate of leakage is 10−3 Pam3s−1 or less. Note that the gas leak test method in which helium gas is used as a probe gas is called helium leak test method.